Research and development aimed at a wider multimedia information society, and more particularly, realization of a ubiquitous service are flourishing. Especially, a device (to be referred to as a memory hereinafter) which is mounted in a network equipment or information terminal to record information is an important key device. The memory mounted in a ubiquitous terminal is required to implement a high-speed operation, long-term holding, environmental resistance, and low power consumption. In addition, a function of inhibiting any erase of stored information even in a power-off state, i.e., nonvolatility is indispensable.
Conventionally, semiconductor devices are widely used as memories. One of the widely used memories is a DRAM (Dynamic Random Access Memory). The unit storage element (to be referred to as a memory cell hereinafter) of a DRAM includes one storage capacitor and one MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor). A voltage corresponding to the state of charges stored in the storage capacity of a selected memory cell is extracted from a bit line as “on” or “off” of an electrical digital signal, thereby reading out stored data (S. M. Sze, “Physics of Semiconductor Devices”, John Wiley and Sons, Inc., 1981, and Fujio Masuoka, “Applied Physics”, Vol. 73, No. 9, p. 1166, 2004).
In the power-off state, however, the DRAM cannot maintain the state of the storage capacitor, and the stored information is erased. In other words, the DRAM is a volatile memory device. Additionally, since the DRAM requires a refresh operation to rewrite data, as is well known, the operation speed is low.
As a nonvolatile memory having the function of inhibiting volatilization of data even in the power-off state, a ROM (Read Only Memory) is well known. However, this memory cannot erase or change recorded data. As a rewritable nonvolatile memory, a flash memory using an EEPROM (Electrically Erasable Programmable Read Only Memory) has been developed (Japanese Patent Laid-Open No. 8-031960, and Fujio Masuoka, “Applied Physics”, Vol. 73, No. 9, p. 1166, 2004). The flash memory is used in various fields as a practical nonvolatile memory.
In a memory cell of a typical flash memory, the gate electrode portion of the MOSFET has a stacked gate structure including a plurality of layers with a control gate electrode and floating gate electrode. The flash memory enables data recording by using a phenomenon that the threshold value of the MOSFET changes depending on the amount of charges stored in the floating gate.
The data write of the flash memory is done on the basis of a phenomenon that hot carriers generated by applying a high voltage to the drain region move over the energy barrier of the gate insulating film. When charges (generally, electrons) are injected from the semiconductor substrate to the floating gate by applying a high field to the gate insulating film and supplying an F-N (Fowler-Nordheim) tunnel current, data is written. The data is erased by removing charges from the floating gate by applying a high field in a reverse direction to the gate insulating film.
The flash memory requires no refresh operation, unlike the DRAM. However, since the F-N tunnel phenomenon is used, the time necessary for the data write and erase is much longer as compared to the DRAM. In addition, when the data write/erase is repeated, the gate insulating film degrades. Hence, the number of times of writes is limited to some extent.
As a new nonvolatile memory different from the above-described flash memory, a ferroelectric memory (to be referred to as an FeRAM (Ferroelectric RAM) hereinafter) using polarization of a ferroelectric or a ferromagnetic memory (to be referred to as an MRAM (Magnetoresist RAM) hereinafter) using the magnetoresistance of a ferromagnetic has received a great deal of attention and been studied extensively. The FeRAM is already put into practical use and therefore expected to replace not only a portable memory but also a logic DRAM if various problems can be solved.
Examples of the ferromagnetic are an oxide ferroelectric (also called a ferroelectric ceramic), a polymer ferroelectric represented by polyvinylidene fluoride (PVDF), and a fluoride ferroelectric such as BaMgF4. In the oxide ferroelectric and fluoride ferroelectric, polarization reverses due to a slight displacement of atoms which are causing the polarization. On the other hand, in the polymer ferroelectric, polarization reverses as individual molecular chains rotate, whose elementary process is a change in conformation (bonding form) of molecular chains which are bonded long by covalent bond.
Oxide ferroelectrics are classified into perovskite ferroelectrics such as BaTiO3 and PbTiO3, pseudo-ilmenite ferroelectrics such as LiNbO3 and LiTaO3, tungsten-bronze (TB) ferroelectrics such as PbNb3O6 and Ba2NaNb5O15, bismuth layer-structured ferroelectrics (BLSF) such as SrBi2Ta2O9 and Bi4Ti3O12, and pyrochlore ferroelectrics such as La2Ti2O7.
Polymer ferroelectrics represented by polyvinylidene fluoride (PVDF) also includes P(VDF/TrFF), i.e., a copolymer of vinylidene fluoride (PDV) and ethylene trifluoride and is prepared by polymerization of a polymer. For further information about ferroelectrics, see “Development and Application of Ferroelectric Materials” edited by Tadashi Shiosaki, CMC Co. Ltd.
Of the above-described ferroelectric materials, the oxide ferroelectrics are mainly used for an FeRAM. Of the oxide ferroelectrics, ferroelectrics having a perovskite structure (to be referred to as perovskite ferroelectrics hereinafter) and, more particularly, lead-based ferroelectrics represented by Pb(Zr,Ti)O3(PZT) are widely used. However, use of lead-containing substances and lead oxides is restricted by the Industrial Safety and Health Law because of concerns about influence on the ecological system and an increase in environmental load. They are therefore being restricted in Europe and U.S.A. from the viewpoint of ecology and pollution control.
Non-lead (lead-free) ferroelectric materials equivalent to the performance of lead-based ferroelectrics have received a great deal of attention on a worldwide basis under the recent necessity for reducing the environmental load. Especially, lead-free perovskite ferroelectrics and bismuth layer-structured ferroelectrics (BLSF) are thought to be most promising. In fact, polarization amounts in these materials are smaller than in the lead-based ferroelectrics, and many unsolved problems still remain in both film formation and process.
FeRAMs that are expected to replace flash memories are mainly classified into stacked memories and FET memories. Stacked FeRAMs are also called 1-transistor/1-capacitor FeRAMs which are categorized in accordance with the structure into FeRAMs with a stacked capacitor as shown in FIG. 127, FeRAMs with a planar capacitor, and FeRAMs with a solid capacitor. The stacked FeRAMs include 1-transistor/1-capacitor FeRAMs and 2-transistor/2-capacitor FeRAMs formed by stacking two 1-transistor/1-capacitor FeRAMs to stabilize the operation.
The stacked FeRAM shown in FIG. 127 comprises, on a semiconductor substrate 12701, a MOS transistor including a source 12702, a drain 12703, and a gate electrode 12705 provided on a gate insulating film 12704. A capacitor including a lower electrode 12711, a dielectric layer 12712 made of a ferroelectric, and an upper electrode 12713 is connected to the source 12702 of the MOS transistor. In the example shown in FIG. 127, the capacitor is connected to the source 12702 by a source electrode 12706. A drain electrode 12707 is connected to the drain 12703. An ammeter is connected to the drain electrode 12707.
This structure has a function of extracting “on” or “off” data by detecting the direction of polarization of the dielectric layer 12712 made of a ferroelectric as a current flowing between the source and drain (channel 12721). The structure has nonvolatility because the polarization of the ferroelectric can be held even without voltage application. In this structure, however, since data is destroyed in the data read, the data must be rewritten, and the speed is therefore low. Additionally, since the area occupied by one element is large, the structure is unsuitable for increasing integration.
In addition to the above-described stacked FeRAMs, FET FeRAMs are expected as FeRAMs of next generation. FET FeRAMs are also called 1-transistor FeRAMs which are categorized in accordance with the structure into MFS (Metal-Ferroelectric-Semiconductor) FeRAMs in which ferroelectric films are arranged in place of the gate electrode of a MOSFET and the gate insulating film in the channel region, MFMIS (Metal-Ferroelectric-Metal-Insulator-Semiconductor) FeRAMs in which a ferroelectric film is arranged on the gate electrode of a MOSFET, and MFIS (Metal-Ferroelectric-Insulator-Semiconductor) FeRAMs as shown in FIG. 128 in which a ferroelectric film is arranged between the gate electrode of a MOSFET and the gate insulating film (Koichiro Inomata, Shuichi Tahara, & Yoshihiro Arimoto, “MRAM Technology—from Fundamentals to LSI Applications”, SIPEC).
In the MFIS shown in FIG. 128, a source 12802 and drain 12803 are provided on a semiconductor substrate 12801. A dielectric layer 12805 made of a ferroelectric is provided on a gate insulating film 12804 arranged between the source and drain. A gate electrode 12806 is provided on the dielectric layer 12805. A source voltage is applied to the source 12802 through a source electrode 12807. An ammeter is connected to the drain 12803 through a drain electrode 12808.
In this FeRAM, polarization of a ferroelectric is applied to the operation of a MOSFET. The FeRAM has a function of creating, by the polarization state, a state wherein a channel 12821 is formed in the semiconductor surface immediately under the gate insulating film 12804 and a state wherein no channel is formed, reading the current value between the source and drain, and extracting the state as “on” or “off” of an electrical digital signal.
In the FET FeRAM, nondestructive read is possible owing to the operation principle because the polarization amount of the ferroelectric does not change even when data is read out. Hence, a high-speed operation is expected. Since the occupation area can be reduced as compared to the 1-transistor/1-capacitor FeRAM, the FET FeRAM is advantageous in increasing integration. Actually, of the 1-transistor FeRAMs, the MFIS FeRAM (FIG. 128) has the gate insulating film between the ferroelectric film and the semiconductor, and for this reason, a polarization reducing field to cancel the polarization amount of the ferroelectric is generated.
To implement the above-described structure, a high-quality high-K dielectric film having a polarization characteristic and orientation is formed on an insulating film generally made of an amorphous material. It is however difficult to form a ferroelectric with a high orientation on an insulating film by using an existing film formation method to be described later. For this reason, in the MFIS FeRAM manufactured by the conventional technique, polarization cannot hold because of the polarization reducing field, and data cannot be held for a long time. If the quality of the insulating film formed on the semiconductor is poor, the polarization amount of the ferroelectric further decreases due to a leakage current generated by the electric field. For these reasons, the data holding period (data life) of the operation of the current MFIS FeRAM serving as a memory remains about 10 days. It is far from commercialization.
In the MFMIS FeRAM, a ferroelectric can be formed on a crystal metal electrode (generally Pt or SrRuO2). Hence, a high-quality film can be formed because the ferroelectric need not be formed on an insulating film, unlike the MFIS FeRAM structure. However, no method to stably form a ferroelectric on a metal has been proposed yet. Since the decrease in polarization by the polarization reducing field generated by the insulating film on the semiconductor poses a problem even in this case, long-term data holding cannot be implemented.
In the MFS FeRAM, since no insulating film is necessary on the semiconductor, the decrease in polarization by the polarization reducing field can be avoided in principle. However, since a ferroelectric film formation method such as a sol-gel process or MOCVD requires a high film formation temperature, the surface of the semiconductor such as Si is oxidized or deteriorated, resulting in an oxide film or many defects on the interface. If an oxide film (interface oxide film) is consequently formed in the interface between the semiconductor and ferroelectric, a polarization reducing field is generated, like the MFIS FeRAM.
If no interface oxide film but a number of defect levels are formed on the interface, the influence of stored charges becomes large, and no accurate memory operation is performed. If the formed ferroelectric film has a poor quality, the leakage current flows in the film, and the polarization characteristic cannot be held for a long time, as is pointed out in many reports.
In the above-described FeRAMs, formation of an oxide ferroelectric on a substrate is important. Various formation apparatuses and various thin film formation methods have been tested until now. Examples are CSD (Chemical Solution Deposition) including a sol-gel process and MOD (Metal-Organic Deposition), MOCVD (Metal-Organic Chemical Vapor Deposition) or MOVPE, PLD (Pulse Laser Deposition), LSMCD (Liquid Source Misted Chemical Deposition), EPD (Electro-Phoretic Deposition), RF-sputtering (also called RF sputtering or magnetron sputtering), and ECR sputtering (Electron Cyclotron Resonance sputtering).
The mainstream of these film formation methods is the CSD called a sol-gel process or MOD. In the CSD, a film is formed by dissolving the matrix of a ferroelectric in an organic solvent and repeatedly applying and sintering the resultant solution on a substrate. As a characteristic feature, a ferroelectric film with a relatively large area can be formed by a simple method. As is reported from many institutions, the CSD can form a ferroelectric film having an arbitrary composition by controlling the composition of the solution to be applied.
There are however problems that it may be impossible to form a film because of poor wettability of the substrate to which the solution is applied and that the solvent used to prepare the solution may remain in the formed film so no high film quality can be obtained. Additionally, in the CSD, the sintering temperature must be higher than the Curie temperature of the ferroelectric film. If the temperature or atmosphere cannot be controlled well, no high-quality film is obtained at all.
Ferroelectric film formation by methods except the CDS have also been tested. For example, the PLD has attracted attention, in which a ferroelectric film having high quality can be formed by sputtering a target of a ferroelectric material with a strong laser source such as an excimer laser. In this method, however, the area of the laser irradiated portion in the target plane is very small, and the material sputtered and supplied from the small irradiated plane has a large distribution. For this reason, in the PLD, a large in-plane distribution is formed in the thickness or quality of the ferroelectric formed on the substrate. There is also a serious problem in reproducibility because the properties change even under the same formation conditions.
However, these properties are suitable for specifically examining conditions. A combinatorial method has received attention as a method of examining the film formation properties by using the above-described properties. However, from the industrial viewpoint, a method capable of forming a large-area film with good reproducibility is essential. It is difficult to industrially use the current PLD.
In addition to the above-described various film formation methods, a sputtering method (to be also simply referred to as sputtering) has received a great deal of attention as a ferroelectric film formation method. Sputtering is a promising film formation apparatus/method because neither dangerous gas nor toxic gas need be used, and a deposited film can have a relatively good surface roughness (surface morphology). In sputtering, a reactive sputtering apparatus/method is considered as a promising apparatus/method for obtaining a ferroelectric film with a stoichiometric composition, in which any oxygen or nitrogen defect is prevented by supplying oxygen gas or nitrogen gas.
In the conventionally used RF sputtering method (conventional sputtering), a compound (sintered body) target is used to deposit an oxide ferroelectric. In the conventional sputtering, however, when an oxide ferroelectric is formed by using argon as an inert gas and oxygen as a reactive gas, oxygen in the ferroelectric film formed on the substrate is not sufficiently captured so no ferroelectric with high quality can be obtained.
For this reason, after the ferroelectric is deposited, the quality of the ferroelectric film formed on the substrate must be improved by executing a heat treatment called annealing in oxygen by using a furnace. In the conventional sputtering, hence, a process called annealing is added, and the manufacturing process becomes complex. In the annealing process, since control is done to obtain a predetermined quality, the conditions such as the temperature must be controlled strictly. Furthermore, annealing may be impossible depending on the material of the formed film.
An example of the method of improving the quality of a sputter film is ECR (Electron Cyclotron Resonance) sputtering. In this method, plasma is produced by ECR. The substrate is irradiated with a plasma flow generated by using the divergent magnetic field of the plasma. Simultaneously, a high frequency or negative DC voltage is applied between the target and ground. Ions in the plasma flow generated by ECR are introduced and made to collide against the target to execute sputtering, thereby depositing a film on the substrate.
In the conventional sputtering, no stable plasma can be obtained without a gas pressure of about 0.1 Pa or more. In the ECR sputtering, stable plasma is obtained at a pressure on the order of 0.01 Pa. In the ECR sputtering, since particles generated by ECR by using a high frequency or high negative DC voltage are caused to strike the target to execute sputtering, sputtering can be done at a low pressure.
In the ECR sputtering, the substrate is irradiated with the ECR plasma flow and sputtered particles. Ions in the ECR plasma flow have an energy of 10 to several ten eV by the divergent magnetic field. In addition, since the plasma is produced and transported at such a low pressure that a gas behaves as a molecular flow, the ion current density of the ions that arrive at the substrate can also be ensured high. Hence, the ions in the ECR plasma give an energy to the material particles which are sputtered and come onto the substrate and also promote the bonding reaction between the material particles and oxygen. Hence, the quality of the deposited film is improved.
As a characteristic feature of ECR sputtering, a high-quality film can be formed at a low substrate temperature. For further information about how to deposit a high-quality thin film by ECR sputtering, see, e.g., Japanese Patent Nos. 2814416 and 2779997 and Amazawa et al., “J. Vac. Sci. Technol.”, B 17, No. 5, 2222 (1999). The ECR sputtering is suitable for forming a very thin film such as a gate insulating film while controlling the thickness well because of the relatively stable film deposition rate. The surface morphology of the film deposited by ECR sputtering is flat on the order of atomic scale. Hence, the ECR sputtering can be regarded as a promising method not only for forming a gate insulating film with high permittivity but also for forming a ferroelectric film necessary for the above-described FeRAM or a metal electrode film.
In some reports, a ferroelectric film using ECR sputtering is also examined. For example, Japanese Patent Laid-Open Nos. 10-152397 and 10-152398 and Matsuoka et al., “J. Appl. Phys.”, 76 (3), 1768 (1994) include reports of a ferroelectric containing barium or strontium. Watazu et al., “Powder and Powder Metallurgy”, No. 44, p. 86, 1997 reports the manufacture of Ba2NaNi5O15. Masumoto et al., “Appl. Phys. Lett.”, 58, 243 (1991).
Predecessors tried to select conditions to form a film made of a ferroelectric material by regarding the ECR sputtering as similar to the conventional sputtering. Hence, even when a ferroelectric film is formed by using the ECR sputtering, no satisfactory ferroelectricity applicable to the FeRAM cannot be obtained so far.
Under the above-described circumstances surrounding the memories, a technique has been proposed (Japanese Patent Laid-Open No. 7-263646), in which the resistance value of a ferroelectric layer 12902 directly formed on a semiconductor substrate 12901 is changed, thereby implementing the memory function, as shown in FIG. 129, instead of implementing a memory by changing the state of a semiconductor (forming a channel) by the polarization amount of a ferroelectric. The resistance value of the ferroelectric layer 12902 is controlled by applying a voltage between electrodes 12903 and 12904.